Neural prosthetic devices are artificial extensions to the body that restore or supplement nervous system function that was lost during disease or injury. Particular success has been realized in the cochlear prostheses development. The devices bypass damaged hair cells in the auditory system by direct electrical stimulation of the auditory nerve. Multiple-electrode cochlear implants are designed to stimulate discrete spiral ganglion cell populations along the cochlea. Stimulating discrete spiral ganglion cell populations in a cochlear implant user's ear is similar to the encoding of small acoustic frequency bands in a normal-hearing person's ear. Thus, it is possible to restore what is commonly thought of as the tonotopic organization of the normal acoustically stimulated cochlea; that is, high frequency tones activate neurons at the base of the cochlea, while low pitch tones stimulate neurons towards the cochlear apex [3-9]. However, the assumption that discrete neural populations can be electrically activated is not always true. It is widely assumed that stimuli applied between closely spaced bipolar electrodes can locally stimulate spiral ganglion cells, whereas widely spaced electrode pairs will lead to broad electric fields and will result in wide areas of neural activation [1, 10]. Nevertheless, for closely spaced electrode pairs at high current levels, a broad region of auditory neurons is activated [10, 11]. Consequently, when two neighboring electrodes are stimulated, a portion of each electric field overlaps, resulting in a population of spiral ganglion cells that are stimulated by both electrodes. If two electrodes stimulate the same neural population, sound sensation encoded via these two electrode contacts might be confused or might even be indistinguishable and this will reduce the number of independent channels of information that can be conveyed to the cochlear implant user. This limitation is based on fundamental physical principles of electrical stimulation that even the best electrode design has not yet overcome.
An important objective in implant electrode design is to maximize the spatial selectivity of stimulation. Several approaches to measure the spatial selectivity of stimulation were reported. In cochlear implant users, impedance measurements on neighboring electrodes were used to estimate the current spread along scala tympani [12-15]. In animal experiments, electrodes were placed at several locations in scala tympani and outside the cochlea. The impedances between each possible pair of electrodes were determined. The current path in the cochlea then was estimated using a lumped element model [3, 16-21]. In another series of experiments, a measuring wire was inserted into the cochlea prior to insertion of the cochlear implant electrode [20, 21]. Stepwise retraction of the measuring electrode allowed the measurement of the potential distribution along the cochlear implant electrode. Again, lumped element models were employed to determine the current path in the cochlea. The common conclusion from all of these experiments was that a large amount of the current injected into the cochlea that was intended for discrete stimulation of the spiral ganglion cells spreads along the scala tympani, thereby non-selectively stimulating broad populations of spiral ganglion cells.
The number of frequency bands required to transmit speech information accurately is an important measurement used in optimization of multiple-electrode stimulation of the cochlea. Shannon et al. [3] and Turner et al. [22] used acoustic models to study the speech information transmitted by fixed filter speech processing schemes. They assessed the optimal number of filter bands to be used as well as the number of cochlear implant electrodes to be stimulated. For quiet listening conditions, a normal-hearing listener could obtain near-normal speech recognition with a four-channel processor. Although these results were confirmed by other groups [23, 24], for noisy listening conditions four channels were not sufficient. It has been estimated that at least twelve independent channels are necessary. Work with cochlear implants demonstrated that speech recognition scores increased with increasing number of electrodes [6, 25-29]. Interestingly, speech recognition scores increased with the number of electrode contacts used, but only up to seven to ten contacts [25]. Normal hearing listeners, in contrast, continued to improve in speech recognition as the number of spectral bands was increased beyond ten. The puzzling aspect of the aforementioned data is that even the best performing cochlear implant users appear to be limited to the equivalent of seven to ten “spectral channels”. Although there are other factors, such as the brand of the cochlear implant device and warping in the spectral-tonotopic mapping, the primary factor limiting speech recognition scores seems to be electrode interaction [7]. The use of laser light to stimulate spiral ganglion cells could be one step towards a more discrete stimulation of the auditory system, thus providing an increased number of independent sub-populations of spiral ganglion cells for speech processing.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.